What is the largest mass that can be measured in the balance

#include <iostream>

using namespace std;

int main() {

 int n, reversedNumber = 0, remainder;

 cout << "Enter a number: ";

 cin >> n;

 int length = to_string(n).length();

 if (length == 4){

   while(n != 0) {

     remainder = n%10;

     reversedNumber = reversedNumber*10 + remainder;

     n /= 10;

   }

   cout << "Reversed Number = " << reversedNumber << endl;

 }

 else{

   cout << "Invalid Input!" << endl;

 }

 return 0;

}

Explanation:

So, it's pretty easy. It finds the remainder to reverse the number and for the invalid input part, you convert the input into a string and then use .length() to find the length. Then you can reverse the number and show whatever output